One key component of any computer system is a device to store data. Computer systems have many different places where data can be stored. One common place for storing massive amounts of data in a computer system is on a disc drive. The most basic parts of a disc drive are an information storage disc that is rotated, an actuator that moves a transducer head to various locations over the disc, and electrical circuitry that is used to write and read data to and from the disc. The disc drive also includes circuitry for encoding data so that it can be successfully retrieved and written to the disc surface. A microprocessor controls most of the operations of the disc drive as well as passing the data back to the requesting computer and taking data from a requesting computer for storing to the disc.
The transducer head is typically placed on a small ceramic block, also referred to as a slider, that is aerodynamically designed so that it flies over the disc. The slider is passed over the disc in a transducing relationship with the disc. Most sliders have an air-bearing surface ("ABS") which includes rails and a cavity between the rails. When the disc rotates, air is dragged between the rails and the disc surface causing pressure, which forces the head away from the disc. At the same time, the air rushing past the cavity or depression in the air bearing surface produces a negative pressure area. The negative pressure or suction counteracts the pressure produced at the rails. The slider is also attached to a load spring which produces a force on the slider directed toward the disc surface. The various forces equilibrate so the slider flies over the surface of the disc at a particular desired fly height. The fly height is the distance between the disc surface and the transducing head, which is typically the thickness of the air lubrication film. This film eliminates the friction and resulting wear that would occur if the transducing head and disc were in mechanical contact during disc rotation. In some disc drives, the slider passes through a layer of lubricant rather than flying over the surface of the disc.
Information representative of data is stored on the surface of the storage disc. Disc drive systems read and write information stored on tracks on storage discs. Transducers, in the form of read/write heads attached to the sliders, located on both sides of the storage disc, read and write information on the storage discs when the transducers are accurately positioned over one of the designated tracks on the surface of the storage disc. The transducer is also said to be moved to a target track. As the storage disc spins and the read/write head is accurately positioned above a target track, the read/write head can store data onto a track by writing information representative of data onto the storage disc. Similarly, reading data on a storage disc is accomplished by positioning the read/write head above a target track and reading the stored material on the storage disc. To write on or read from different tracks, the read/write head is moved radially across the tracks to a selected target track. The data is divided or grouped together on the tracks. In some disc drives, the tracks are a multiplicity of concentric circular tracks. In other disc drives, a continuous spiral is one track on one side of disc drive. Servo feedback information is used to accurately locate the transducer head. The actuator assembly is moved to the required position and held very accurately during a read or write operation using the servo information.
The actuator is rotatably attached to a shaft via a bearing cartridge which generally includes one or more sets of ball bearings. The shaft is attached to the base and may be attached to the top cover of the disc drive. A yoke is attached to the actuator. The voice coil is attached to the yoke at one end of the rotary actuator. The voice coil is part of a voice coil motor which is used to rotate the actuator and the attached transducer or transducers. A permanent magnet is attached to the base and cover of the disc drive. The voice coil motor which drives the rotary actuator comprises the voice coil and the permanent magnet. The voice coil is attached to the rotary actuator and the permanent magnet is fixed on the base. A yoke is generally used to attach the permanent magnet to the base and to direct the flux of the permanent magnet. Since the voice coil sandwiched between the magnet and yoke assembly is subjected to magnetic fields, electricity can be applied to the voice coil to drive it so as to position the transducers at a target track.
The voice coil motor pivot friction has an increasing impact on hard disc drive servo control. This is generally true in cases where the voice coil motor uses a ball bearing design, and where the hard disc drive has high tracks per inch. In the case of high tracks per inch, the width of a track is so minute, that the movement of the bearing may also become minuscule, and that the friction at these low velocities can become significantly dominant. This friction dominance at low velocity displacement due to high tracks per inch has become extremely important in low cost desk top hard disc drive servo controller design. This friction at low velocities introduces a nonlinear dynamics on the hard disc drive servo controller. It is generally easier to design disc drive controllers for linear systems. Small nonlinearities can often be neglected in the design of controllers, or even approximated by linearizations. It is generally difficult to include the nonlinear dynamics introduced by the voice coil motor pivot friction in the design of the disc drive controllers. In general at high velocities the voice coil motor friction can be characterized as linear, but at low velocities the voice coil motor pivot friction cannot be linearized. If a linear design is used for the controller at low velocities when the voice coil pivot friction is present, bias estimator in the controller can estimate abnormally high bias values, which can result in unexpected behavior of the hard disc drive during a seek operation, such as sample errors resulting in seek failures, and the inability of the voice coil motor to park against the latch as the bias estimator may have diverged to a point where the bias force may be equivalent to the parking current in the opposite direction.
The voice coil motor pivot friction can vary significantly with large variations in the manufacturing tolerances in the hard disc drive components. Currently there are no reliable, accurate and cost effective methods to characterize the voice coil pivot friction.
What is needed is a reliable, accurate, and cost effective method of characterizing and salvaging disc drives having acceptable voice coil motor pivot friction levels.